The deterioration of human bone, whether it be due to various degenerative diseases such as osteoarthritis or through bone and/or joint related injuries, is a continuing problem which has resulted in the development of a wide range of implantable orthopedic devices. The need for such implantable devices has encouraged the medical community to come up with new and better bone and/or joint replacements which attempt to mimic the actual structure and motion of the natural articles, while being both stable in biological environments and durable with regard to the stresses and strains put on them by bodily motions. These implants, although constructed to be as simple as possible, may have a plurality of interacting parts. Through repeated use and even from extended residence in the body, these interacting parts tend to break down, causing pain or discomfort, limiting the effective range of motion of the device, and even requiring the replacement of the implant.
The present invention is concerned with the implant bearing surfaces of these devices and means to secure the material that is used in the construction of these surfaces to the implant. It is these implant bearing surfaces which, because of their intimate contact with each other, must be made as smooth and frictionless as possible, especially in joints that demonstrate the widest range of motion. For example, consider the hip joint which is a ball-and-socket joint and as such has a relatively wide range of motion. The joint is composed of two main members, a rounded femoral head and a cup-like socket or acetabulum located in the pelvis, both of which have surfaces that continually abrade each other as the joint moves. Deterioration of the femoral head and/or acetabulum requires replacement of one or both members using an appropriate prosthetic device. These prosthetic devices have been improved over the years, but in general still lack a smooth and frictionless bearing surface which does not produce debris from ball-and-socket motion along the areas of contact, and secure mounting means that eliminates motion at the interface of the implant shell component and bearing insert.
Current technology provides implant bearing materials which are composed of an ultra high molecular weight polyethylene (UHMWPE) machined component which is anchored into the bone with bone cement. Alternatively, the polyethylene component may be assembled to a solid metallic substrate for bone ingrowth. The problem with these polyethylene components, however, involves their attachment either directly to bone or to prosthetic devices. In the case of an acetabular cup, for example, it has proven to be difficult to develop secure mounting means to attach the polyethylene bearing insert to the shell of the acetabular cup.
Many methods and devices have been developed to improve the fixation of the implant in the body so that the implant and its bearing material become as permanent as possible. For example, in interference fit situations where bone quality and patient criteria constraints are met, the polyethylene component is typically secured by a locking mechanism which is either built into or placed adjunct to the metallic cup. The plastic insert may be secured within the metal cup in a variety of methods which include the use of retaining rings, press fitting or force fitting the plastic insert into the interior of the metal cup and/or thermally fitting the plastic insert into the interior of the metal cup. The metal cup is then secured to the patient's acetabulum in any one of a number of commonly practiced techniques. The problem with this arrangement, however, is that the locking grooves which hold the polyethylene surface to the metal cup typically allow for both micro and macro motion at the interface of the polyethylene/metal which, in turn, can cause debris in and around the polyethylene/metal boundary. This polyethylene debris is known to cause bone lysis and is thus severely damaging to surrounding tissue. Additionally, the solid metal cup offers considerable rigidity to the structure, and can adversely affect the natural remodeling of bone.
Another well known method to secure polyethylene implant bearing materials in the body is through the use of cements such as polymethylmethacrylate that anchor the implant to the remaining bone structure. In these cemented situations, where bone quality or patient limitations may not allow an interference fit device to be stable, this bone cement is used as an immediate implant stabilizer by filling all bone voids and securing the device. However, the fact that the polymethylmethacrylate will not bond directly to polyethylene requires the addition of geometric locking grooves for cement mechanical retention. Also, even for those situations where this cement is useful and appropriate, it has a number of distinct disadvantages including a waiting time necessary for the cement to harden, the release of heat from the hardening process which could damage surrounding tissue, and the fact that the presence of this cement prevents bone ingrowth into the prosthesis.
Yet another way which has been used to improve the permanence of implants in the body is to construct the implants with porous outer surfaces so as to receive an ingrowth of body tissue. These implant devices can be made of either thermoplastics or metallic materials. Examples of prosthetic devices constructed with porous thermoplastics appear in U.S. Pat. Nos. 4,164,794 and 4,756,862 both entitled "PROSTHETIC DEVICES HAVING COATINGS OF SELECTED POROUS BIOENGINEERING THERMOPLASTICS" which were issued to Spector et al. and assigned to the Union Carbide Corporation. These devices are composed of an inner functional component and an outer foamed or sintered porous coating of thermoplastics. The thermoplastic coating provides a region where long-term bone fixation is made possible by tissue ingrowth. An example of a porous metallic implant is described in U.S. Pat. No. 5,456,723 entitled "METALLIC IMPLANT ANCHORABLE TO BONE TISSUE FOR REPLACING A BROKEN OR DISEASED BONE" which was issued to S. G. Steinemann et al. and assigned to Institut Straumann AG. The metallic implant discussed therein provides a porous outer coating for tissue ingrowth and may be made with a plurality of inert metals such as titanium, zirconium, niobium and tantalum. The micro-roughness provided by an acid etching step provides a heavily pitted surface having a very large surface area which is ideal for tissue ingrowth. An implant device constructed with a combination of metallic/nonmetallic parts is also possible as demonstrated in U.S. Pat. No. 5,443,512 entitled "ORTHOPEDIC IMPLANT DEVICE" which was issued to J. E. Parr et al. and assigned to Zimmer, Inc. This device combines the above-mentioned tissue growth-encouraging advantage of a porous metallic layer with the load stability and flexibility of a polymer core. Moreover, the porous metallic layer is melted into the polymer casing so that adhesion and mechanical interlock are achieved without the aid of bone cements.
While the above-mentioned implants have been used successfully for the replacement of human skeletal parts, they in general do not address the problems associated with bearing interfaces. In order to be both functional at bearing interfaces and suitable for long-term residence in the body, such a device needs one surface that is porous and can incorporate natural bone ingrowth for permanent fixation, and another surface which is smooth and frictionless yet durable enough to withstand years of abrasion without creating debris.
Currently there exists an acetabular cup orthopedic implant device which consists of a screen grid-type metallic material hot pressed to polyethylene. The cementless acetabular press-fit cup is constructed with an outer titanium mesh which is created with superimposed layers of screens having a specific pore size and known porosity volume which are welded together in a parallel arrangement to each other. An inner polyethylene bearing material is hot pressed into the titanium mesh shell to form an acetabular cup. The result is an implant device which has both a rigid outer shell that is capable of incorporating bone ingrowth, and a bearing insert which defines a smooth surface for use with a rounded femoral prosthesis end piece. The hot pressing technique, however, does not optimize the mechanical and wear properties of the polyethylene. This is because the polyethylene cup and the titanium mesh material are made separately and then fixed together in a heating process. Because the heat distribution of the hot pressing technique is not uniform, the polyethylene does not melt uniformly, thus causing surface defects and ultimately damaging the surface characteristics of the bearing interface. In addition, the grid-type metallic material is created with superimposed layers of screens resulting in a material and pore shape that does not accurately mimic the characteristics of natural cancellous bone for optimal natural bone ingrowth.
A recently developed, lightweight, strong, porous metal structure mimicking the microstructure of natural cancellous bone which also acts as a matrix for the incorporation of bone is described in U.S. Pat. No. 5,282,861 entitled "OPEN CELL TANTALUM STRUCTURES FOR CANCELLOUS BONE IMPLANTS AND CELL AND TISSUE RECEPTORS" issued to R. B. Kaplan and assigned to Ultramet. The entire disclosure of the '861 patent is incorporated herein by reference. The material disclosed in the '861 patent is available from the Implex Corporation under the tradename HEDROCEL. This material consists of a three dimensional network of pores which form continuous, uniform channels with no dead ends. The material and has a much lower modulus than a pure metallic implant, and has a significantly better pore size and shape distribution than prior art materials. This intricate network of interconnected pores provides optimal permeability and a high surface area to encourage tissue ingrowth, vascularization, and deposition of new bone, while also allowing for the interdigitation of bone cement for those situations that require it. HEDROCEL, although ideal for the construction of orthopedic implants requiring mechanical integrity (as in load bearing applications), is not suitable for use at implant bearing surfaces unless post-processed to include a layer of material such as polyethylene.
It is, therefore, a primary objective of the present invention to provide a composite material for use in medical implants consisting of a first material that is suitable for bone ingrowth or cement interdigitation and a second material that is suitable for implant bearing surfaces, the interface of the two materials consisting of the second material completely interdigitated into a region of the first material such that the second material remains fixed to the first material and does not create debris.
It is a further object of the present invention to provide an acetabular cup prosthesis constructed with the composite material that substantially overcomes the problems associated with prior art acetabular cups.